TRANSCRIPTIONAL REGULATION OF THE HUMAN CHOLESTEROL 7-ALPHA-HYDROXYLASE GENE (CYP7A) IN HEPG2 CELLS

A stable HepG2 cell line harboring a human cholesterol 7 alpha-hydroxy lase (CYP7A) minigene/luciferase reporter gene construct was selected for studying transcriptional regulation of CYP7A gene promoter. Insuli n and phorbol 12-myristate-13-acetate (PMA) strongly repressed the pro moter activity as measured with luciferase activity expressed in the c ells. The promoter activity of the 5' progressive deletion/luciferase reporter gene constructs was studied in a transient transfection assay in HepG2 cells. PMA re presses the promoter activity and the response elements were localized in the -184/-151 and -134/-81 regions. Insuli n also represses the promoter activity and response element was mapped in the -298/-81 region. Surprisingly, glucocorticoid receptor (GR) st rongly inhibited promoter activity in the presence of dexamethasone, a nd response elements were localized in the -298/-151 and the -150/+24 regions. Thyroid hormone receptor also repressed promoter activity and response elements were localized in the -150/+24 and upstream regions . Cotransfection of CYP7A chimeric constructs with an expression vecto r carrying liver-enriched transcription factor HNF3 alpha stimulated t he reporter gene activity, but cotransfection with GR plasmid interfer ed with the HNF3 alpha-stimulated activity possibly through competitio n for binding to overlapping GR/HNF3 binding sites.jlr Thus, human cho lesterol 7 alpha-hydroxylase gene promoter is strongly repressed by in sulin, PMA, and steroid/thyroid hormones and results in the low level of cholesterol 7 alpha-hydroxylase expression in the human liver.